The present invention relates to a liquid crystal device for use in light-valves for flat panel displays, paper displays, projection displays, printers, etc. The present invention also relates to a mesomorphic functional material suitable for constituting the liquid crystal device and a liquid crystal apparatus including the liquid crystal device.
The most popular and extensively used display may be CRTs (cathode ray tubes) which have been heretofore used for displaying motion pictures of television and video tape recorders or as monitor displays for personal computers. Based on the operation characteristic, however, the CRT is accompanied with difficulties such that the recognizability of a static image is lowered due to flickering and scanning fringes caused by an insufficient resolution, and the fluorescent member is deteriorated due to burning or sticking. Further, the CRT has generally consumed a larger electric power, thus being required to reduce its power consumption. Further, the CRT structurally has a large rearward space behind the display surface to restrict facilitates for information equipment having the CRT, so that the space economization in offices at home may be obstructed thereby.
As a type of device solving such problems of the CRT, there has been known a liquid crystal device, including a type using a twisted nematic (TN) liquid crystal as disclosed by M. Schadt and W. Helfrich, xe2x80x9cApplied Physics Lettersxe2x80x9d, Vol. 18, No. 4 (Feb. 15, 1971), pp. 127-128.
By using a liquid crystal material of the TN-type, VA (vertical alignment)-mode, IPS (in plane switching)-mode, etc., an active (TFT)-type liquid crystal device has been developed and commercialized in recent years, wherein each pixel is provided with and driven with a TFT (thin film transistor). As a result, the problem of crosstalk has been solved. Further, along a rapid progress in production technology in recent years, TFT-type display panels using such a liquid crystal device of 1-13 inch-size have been produced with good productivity.
However, the above-mentioned liquid crystal devices are ordinarily used as a display panel in combination with a backlight (device) by optically modulating a transmitted light passing through the liquid crystal device. Accordingly the backlight for the liquid crystal device is required to emit a strong light. Further, a consumption power of liquid crystal display apparatus is largely occupied by the backlight. Even when a lithium ion-secondary battery is used for such a liquid crystal display apparatus a continuous (successive) operation time for, e.g., mobile computing is approximately several hours at the best. Thus, if backlight devices for various liquid crystal devices can be omitted, low power consumption for many information equipment and office equipment is realized, thus leading to suppression of global warming and a terrestrial environment protection.
In the circumstances, a low power consumption-type reflection liquid crystal device without using a backlight has been developed but still leaves room for improvement in its characteristics at present. Further, various products using a projection-type liquid crystal device as a projector have been commercially available from electrical equipment manufacturers as a large picture-size display. In the field of such a liquid crystal projector, however, a further improvement in brightness (luminance) and/or contrast is required.
In order to provide a high-brightness liquid crystal device without using a polarizer, light scattering-type liquid crystal devices, such as one of a polymer-dispersed type and one of a polymer network-type have been developed and proposed (e.g., xe2x80x9c""93 Eurodisplayxe2x80x9d, p. 397-). However, these liquid crystal devices are still desired to improve driving characteristics, scattering performance and other characteristics.
Japanese Laid-Open Patent Application (JP-A) 09-243984 describes a light scattering-type liquid crystal device using a combination of dendrimer or a discotic mesomorphic compound with a rod-shaped mesomorphic compound. However, the discotic mesomorphic compound and the rod-shaped mesomorphic compound have mutually similar chemical structures, thus providing an insufficient phase separation state. As a result a light-scattering performance largely affected by the phase separation state is liable to become insufficient and a switching (optical response) ability of the rod-shaped mesomorphic compound is liable to be impaired.
In view of the above-mentioned problems, an object of the present invention is to provide a liquid crystal device of low power consumption, high brightness and high performance as an optical modulation device or a display device.
Another object of the present invention is to provide a mesomorphic functional material excellent in properties and suitable for a liquid crystal material of the liquid crystal device.
A further object of the present invention is to provide a liquid crystal apparatus including the liquid crystal device.
As a result of our study, we have found that it is possible to realize a liquid crystal device providing a high light-scattering state effectively utilizing a difference in refractive index characteristic between a discotic mesomorphic compound and a rod-shaped mesomorphic compound (as described later as to Table 1) by placing the mesomorphic compounds in such a state that the discotic mesomorphic compound and the rod-shaped mesomorphic compound essentially having mutually different molecular structures leading to different refractive index anisotropies are co-present in a phase separation state.
Herein, the term xe2x80x9cphase separation statexe2x80x9d means a state such that under observation through an optical polarizing microscope, mutually phase separated extures of the liquid crystal layer is observable or confirmable. Specfically, this condition is fulfilled when a domain size of each phase is at least a light wavelength level, typically 1-2 xcexcm or larger. In some cases, the phase separation state may also be confirmed by effecting thermal analysis using, e.g., a DSC (differential scanning calorimeter) with respect to respective mesomorphic phases to determie an individual phase transition behavior.
According to the present invention, there is provided a liquid crystal device, having a liquid crystal layer of a liquid crystal composition comprising a discotic mesomorphic compound and a rod-shaped mesomorphic compound, wherein the discotic mesomorphic compound and the rod-shaped mesomorphic compound are co-present in the liquid crystal layer in a mutual phase separation state.
In a preferred embodiment, in order to provide the above-mentioned phase separation state, a polymeric discotic mesomorphic compound having a recurring unit comprising a discotic mesomorphic molecular unit is used as the discotic mesomorphic compound.
In another preferred embodiment, in order to form a high light-scattering state, directors of the discotic mesomorphic compound and the rod-shaped mesomorphic compound in the liquid crystal layer are oriented in an identical direction in at least one alignment state.
In a further preferred embodiment, as the rod-shaped mesomorphic compound, a mesomorphic compound having a memory characteristic or nematic phase or a mesomorphic compound in a polymerized state (polymeric mesomorphic compound) is used.
According to another aspect, the present invention provides a liquid crystal composition or mixture as a liquid crystal material for forming the liquid crystal layer in the above-described preferred embodiments.
In this case, as a preferred liquid crystal material, a liquid crystal mixture comprising a polymerizable discotic mesomorphic compound and a rod-shaped mesomorphic compound is used.
According to still another aspect of the present invention, there is provided a liquid crystal apparatus comprising the above liquid crystal device and a drive means for driving the liquid crystal device.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.